Thanks Rolf, I am not sure this is what you meant but I changed it to:

 while (<FH>)
          {
          if (/(\S+):(\S+).*\n/)
               {
               $var1 = "$1";
               $var2 = "$2";
               s/$var1/$hash{$var1}_$var/g;
               }
           }
[download]

and now I get a different Use of uninitialized value error, which seems better?:

"Use of uninitialized value within %hash in concatenation (.) or string at test_perl.pl line 41"

I don't understand your comment about nested regexes, could you elaborate? Thanks in advance for any help